add option to use clock_gettime() in perf example

This is done to support architectures where `rdtsc()` is not implemented.
In case it is available use `clock_gettime()`. Otherwise still fall back
to use `clock()`.

The first approach was to always use `clock_gettime()` if it is available
but the overhead it brings compared to `rdtsc()` is high, so I chose to
only try to use it if `rdtsc()` is not implemented.

Signed-off-by: Steffen Jaeckel <jaeckel-floss@eyet-services.de>

configure.ac

@@ -86,6 +86,7 @@ AX_VALGRIND_DFLT([helgrind], [off])
 AX_VALGRIND_DFLT([sgcheck], [off])
 AX_VALGRIND_CHECK
 
+AC_SEARCH_LIBS([clock_gettime], [rt], AC_DEFINE(HAVE_CLOCK_GETTIME,1))
 AC_SEARCH_LIBS([socket], [network socket])
 AC_CHECK_FUNCS([snprintf vsnprintf], [], [have_snprintf=no])